1. The Field of the Invention
The present invention relates to the manufacture of semiconductor devices. More particularly, the present invention is directed to methods employing vapor phase etchants for etching oxides of silicon during the manufacture of a semiconductor device such that the selectivity of the etchant is low. The methods of the present invention are also useful in removing contamination other than silicon oxides typically encountered in semiconductor manufacturing process flows, such as polymer residues, while providing low selectivity.
2. The Relevant Technology
In the continuing quest for ever denser DRAM devices, the problem of properly forming capacitors for each memory cell becomes increasingly acute. The problem is due to the need of providing both sufficiently large capacitance to preserve a charge between refresh cycles and sufficiently small size to allow further reductions in circuit dimensions. Dimensional tolerances in capacitor formation have thus tended to become a yield-limiting and density-limiting factor in DRAM devices.
Clean processes are a significant source of decreased dimensional control in the formation of capacitor structures. Removal of native oxides and other types of oxide contamination is required at various steps during capacitor formation. A short dip in a dilute solution of hydrofluoric acid (HF), such as a 100:1 volumetric ratio of water to 49% HF solution, is typically employed for this purpose. Problems arise because the dilute HF solution also significantly and even preferentially attacks doped silicon dioxide such as BPSG in which the capacitor structures are formed and defined, resulting in decreased control of critical dimensions associated with the capacitor. A less selective process is thus needed to remove native oxides and other types of oxide contamination during capacitor formation without excessively attacking doped silicon dioxide such as BPSG. As doped silicon dioxide is etched, an unacceptable level of residue can be built up.
When doped silicon dioxide, such as BPSG, is etched, HF and H.sub.2 O accumulate on the surface of the BPSG to form H.sub.3 PO.sub.4 and P.sub.2 O.sub.5, the later being from the phosphorous dopant in the BPSG. As the P.sub.2 O.sub.5 is etched, phosphoric acid is produced. This etching byproduct is detrimental in that the phosphorous remains on the semiconductor wafer. Phosphorous on the semiconductor wafer can form crystals which can then flake off and cause particulate contamination, or cause problems with photolithography. Additionally, the phosphoric acid residue can diffuse into the BPSG and then later out-diffuse, depending on ambient conditions, so as to selectively cause corrosion should the out diffusion occur in the presence of a corrosion prone metal. Consequently, it is best to avoid phosphoric acid build up on the semiconductor wafer.
A dilute HF solution is also typically employed to remove native oxide or other oxide contamination at process steps during which a refractory metal silicide such as titanium suicide is exposed to the solution. This may occur, for example, in a clean step prior to the formation of spacers around a gate stack that includes a refractory metal silicide layer, or during a clean step prior to filling a contact to a gate stack that includes a refractory metal silicide. As dimensions of gate stacks decrease, this use of dilute HF solution creates problems because the refractory metal silicide layer is preferentially etched by the dilute HF solution, such that where dimensional tolerances are small, the refractory metal silicide layer may be seriously damaged or even completely destroyed. A less selective process is thus needed to remove native oxide and other types of oxide contamination during gate formation and contact formation without excessively attacking refractory metal silicides.
Wet etching processes use a large quantity of chemicals that require clean-up procedures incident to their use. In addition to consumption of water by wet etching processing, wet etching may also have a detrimental environmental impact. An economical etching process that avoids such problems know to wet etching would be desirable.